QUARTZ, FELSPAR, MICA, HORNBLENDE, AUGITE. 85 



of the granitic rocks. They contain, in inexhaustible 

 abundance, all the mineral elements necessary to the 

 growth of a plant, potash, soda, lime, magnesia, iron and 

 manganese, in the condition of silicates. 



290. The following table will show this to be true. 

 Remember that granite, gneiss and mica slate, are com 

 posed of mica, quartz, and felspar; syenite, of quartz, 

 felspar and hornblende ; trap-rock, of augite, felspar and 

 hornblende ; greenstone, of felspar and hornblende ; and 

 porphyry, almost entirely of felspar. 



In one hundred parts, there arc, in these .different 

 minerals, about these proportions. For great exactness, 

 see Dana s Manual. 



Silica. Alumina. Potash. Magnesia. Iron. Lime. 

 In Quartz, . . - . 100. 



Felspar, . \ . 67. 19. 14. 



Mica, . ; . 46. 14. 10. 10. 20. 



Hornblende, . . 59. 20. 7. 14. 



Augite, ... 53. 8. 17. 22. 



291. Quartz is silica nearly pure. Felspar is a silicate 

 of alumina and potash. Mica is a silicate of alumina and 

 potash, and of magnesia and iron. Hornblende is made 

 of silicates of magnesia and lime, with iron ; and augite, 

 of silicates of lime and magnesia, with a larger propor 

 tion of iron. In some kinds of felspar soda takes the 

 place of potash. 



292. How is a light, sandy soil, possessing the mineral 

 elements of fertility, to be managed, that it may become 

 fertile ? The first thing to be done is to render it capable 

 of absorbing moisture, carbonic acid, oxygen, and ammo 

 nia, and of retaining them so as to give them out to the 

 roots of plants as they are wanted. This is done by 

 mixing with it clay, which has these properties in a very 

 considerable degree. 



